Nuclear Magnetic Resonance (NMR) Imaging, or Magnetic Resonance Imaging (MRI) as it is commonly known, is a non-invasive imaging modality that can produce high resolution, high contrast images of the interior of the human body. MRI involves the interrogation of the nuclear magnetic moments of a subject placed in a strong magnetic field with radio frequency (RF) magnetic fields. A MRI system typically comprises a fixed magnet to create the main strong magnetic field, a gradient coil assembly to permit spatial encoding of signal information, a variety of RF resonators or RF coils as they are commonly known, to transmit RF energy to, and receive signals emanating back from, the subject being imaged, and a computer to control overall MRI system operation and create images from the signal information obtained.
The design of RF resonators operating in the near-field regime plays a major role in the quality of magnetic resonance imaging. Over the past few decades, the designs of RF resonators have significantly evolved from the simple solenoid coils of wire that were typically used. The volume coil, such as for example the saddle coil, hybrid birdcage or TEM, has been a popular choice of MRI transmitter and/or receiver for large fields of view (FOV) due to the ability to match it to 50Ω impedance transmit and receive components and its homogeneous sensitivity profile. Although these volume coils operate well at low magnetic field strengths, they become less effective in the high field regime i.e. at magnetic field strengths greater than 3 T. As the static magnetic field strength used in MRI increases, the wavelength of the associated Larmor RF approaches the dimensions of the volume coil and volume of interest (VOI). Several imaging problems arise in this full wavelength regime namely, increased radiation losses, increased local and global specific absorption rate (SAR), and dielectric resonance effects that create both inhomogeneous images and signal loss.
By using surface coils to receive, one can reduce the detrimental effects of dielectric resonance on signal homogeneity commonly observed with volume coils at higher field strengths (>3 T). Surface coils can also be designed to target specific VOIs thereby to reduce unnecessary power deposition within the patient. Furthermore, the increased sensitivity of surface coils for reception, in comparison to volume coils, presents an opportunity for increased image signal-to-noise ratio (SNR) in both receive-only and transmit and/or receive (“transceive”) modes.
With the advent of fast parallel imaging techniques such as SMASH, SENSE and transmit SENSE, there exists a greater need for flexible placement and combination of multiple receiver and/or transmitter RF coils. The sensitivity profiles of these multiple RF coils are required for and influence the efficiency of these fast parallel imaging methods. Fast parallel imaging techniques provide the ability to remove or unfold aliasing artefacts in under-sampled images. This ability to un-alias images provides a means to increase temporal resolution. Images that may have been impossible to acquire within the time constraints of breath hold techniques may be realizable through the reduction of motion artifacts.
There are several design approaches for both volume and surface coils that have been implemented to acquire multiple sensitivity profiles with considerable success. For example, the degenerate mode birdcage has been shown to be useful in sensitivity encoding although receive-only surface coil arrays provide higher SNR and are more suitable for high field MRI. The predominant impediment to surface coil array design is however, the strong magnetic coil-to-coil coupling.
There are several design approaches available to reduce this magnetic coil-to-coil coupling, including preamplifier decoupling, strip transmission line arrays, overlap geometries, and capacitive decoupling networks
Magnetic coil-to-coil coupling can be substantially eliminated between two (2) neighboring surface coils using a unique overlap of surface coils. Unfortunately, the resultant overlapping sensitivity profiles are less than ideal for fast parallel imaging techniques, which are more effective when sensitivity profiles of individual surface coils do not overlap.
Alternatively, the effect of magnetic coil-to-coil coupling on both nearest neighbor and next nearest neighbor surface coils can be reduced using a decoupling method employing low impedance preamplifiers. Unfortunately, low (or high) impedance preamplifiers are not generally available off-the-shelf and are therefore, more complex, expensive and time consuming to implement. Furthermore, low input impedance RF power amplifiers are not widely commercially available off-the-shelf, thereby practically limiting this coil-to-coil magnetic coupling reduction technique to receive-only applications. More complicated capacitive ladder networks have been employed to reduce magnetic coil-to-coil coupling at lower field strengths. However, considerable electric field loss and strong coupling between lattice networks limits their application at higher field strengths.
Other coil-to-coil coupling reduction techniques have also been considered. For example, U.S. Pat. No. 5,973,495 to Mansfield discloses a method and apparatus for eliminating mutual inductance effects in resonant coil assemblies in which a plurality of coils is situated in sufficiently close proximity to create small mutual inductances between the coils. Mutual inductances are evaluated using a T star or other transformation of the relevant parts of the circuit thereby to isolate the inductances in such a way that series capacitances may be introduced to tune out the mutual inductances at a common frequency, reducing the coil array to a synchronously tuned circuit. Unfortunately, this design requires a common ground resulting in electric field losses and requires a common connection between all of the coils, which is geometrically restrictive.
U.S. Pat. No. 6,788,059 to Lee et al. discloses an RF detector array based on a microstrip array decoupling scheme. The detector array comprises a plurality of conductive array elements that is substantially parallel to a conductive ground plane and a plurality of capacitors. At least one capacitor is shunted from each conductive array element to the ground plane to adjust a corresponding electrical length of each conductive array element. A combination of each respective conductive array element, at least one corresponding capacitor and the ground plane forms a resonator that resonates at a selected frequency. A decoupling interface and a plurality of matching boxes match each decoupled strip to a selected impedance.
U.S. Patent Application Publication No. 2002/0169374 to Jevtic discloses a capacitive ladder network to achieve next nearest neighbor (NNN) coil-to-coil decoupling. Unfortunately, this ladder network is complex and appears to be limited to low field MRI applications, as considerable electric field losses, and strong coupling between lattice networks would limit its application at high field strengths.
U.S. Patent Application Publication No. 2003/0184293 to Boskamp et al. discloses a multiple channel array coil for magnetic resonance imaging, that similar to Lee et al., is based on a microstrip array decoupling scheme. The array coil includes a plurality of conductive strips formed within a dielectric medium. The conductive strips are arranged into a generally cylindrical configuration with each of the strips having a length selected to cause each of the conductive strips to serve as a resonator at a frequency corresponding to a proton MRI frequency. The cylindrical configuration of the conductive strips forms a multiple channel, volume resonator in which each of the conductive strips is isolated from the remaining strips.
As will be appreciated, there exists a need for a surface coil array that is capable of transmit and/or receive operation for use in fast parallel imaging techniques such as SENSE imaging. There is a further need for a surface coil array that is capable of operation at both low and high magnetic field strengths without succumbing to SAR limitations. There is a further need for a surface coil array that can operate with conventional 50Ω transmit and receive components including preamplifiers and less expensive low power amplifiers, while maintaining the SNR benefits of receive-only surface coils.
It is therefore an object of the present invention to provide a novel surface coil array for magnetic resonance imaging and spectroscopy.